Very low frequency resistance fluctuations in thin films of La_0.67Ca_0.33MnO₃ with quenched disorder

Abstract

In this paper we report the appearance of very low frequency (<1 mHz) resistance fluctuations (noise) in strain relaxed rare-earth perovskite manganite films of La_0.67Ca_0.33MnO₃ with quenched disorder grown on SrTiO₃ (STO). The films show high degree of orientation and extremely low 1/f noise. The observed spectral power clearly consists of two components: a Lorentzian part (arising from discrete fluctuators) that peaks near the ferromagnetic transition and a broadband 1/f part that is essentially temperature independent. The noise power in the films with quenched disorder, interestingly, is orders of magnitude less than that seen in nearly strain free and uniformly strained thin films (thickness ≤50 nm) grown on NdGaO₃ and STO, respectively. The discrete fluctuators seen in the films with quenched disorder are thermally activated with a high activation energy, ∼0.7 eV. Strain accommodation has been suggested as one of the likely mechanisms that leads to such a high thermal activation energy. The resistance fluctuation can be changed even by a small magnetic field (≤0.1 T). The relative variance of the fluctuation has been observed to be proportional to the measured magnetoresistance (dR/dH). We propose that the fluctuations arise from magnetization fluctuations, which couple to the resistance fluctuations by the magnetoresistance. The magnetization fluctuations arise from magnetic domains as well as from the fluctuating magnetization of the coexisting phases.